1. Field of the Invention
The present invention relates to a focus servo control device, and more particularly, to a focus servo control device which shortens the time needed to prepare for focus servo control by shortening focus gain adjustment time.
2. Description of the Related Art
In a compact disc (CD) player, the focal depth of an object lens for focusing a laser beam on the surface of a disc is approximately only .+-.0.1.mu.m. Therefore, the CD player is provided with a focus servo control device which generates a focus error signal having a voltage value corresponding to the distance between the object lens and the disc, performs focus servo control based on the focus error signal, and controls the position of the object lens with respect to the surface of the disc so that the above distance is constant, in other words, so that the surface of the disc accords with a focal plane.
FIG. 4 schematically explains the focus servo control in which a focus error signal FE is generated by using a photodetector PD divided into four diodes in astigmatism. (1) In focus (when the object lens is focused on the surface of the disc), a beam image on the photodetector PD has a circular shape as shown in FIG. 4(a). (2) When the object lens is too close to the reflecting surface of the disc, a beam image on the photodetector PD is in the shape of a longitudinal ellipse as shown in FIG. 4(b). (3) When the distance between the object lens and the reflecting surface is too long, a beam image on the photodetector PD is in the shape of a laternal ellipse as shown in FIG. 4 (c).
Therefore, if a composite current of first and third photodiodes D1 and D3 is converted into voltage by an I-V converter (current-voltage converter) 1 and input to a + terminal of a subtracting circuit 3 in the next stage, and a composite current of second and fourth photodiodes D2 and D4 is converted into voltage by an I-V converter 2 and input to a-terminal of the subtracting circuit 3, a focus error signal FE in accordance with the distance between the disc reflecting surface and the object lens is output from the subtracting circuit 3 as shown in FIG. 4(d). Since the focus error signal FE is 0 in focus, the focus servo control is performed so that FE is 0, in other words, so that the object lens is focused on the disc reflecting surface.
In order to allow the focus servo control device to perform focus servo control, it is necessary that the distance between the object lens and the disc reflecting surface be somewhat close to the focal depth. It is thus necessary to move the object lens to a position where the focus servo control can be performed when the disc is set on a turntable. Therefore, a focus search signal which is a triangular signal of approximately 1/2 Hz is generated, the object lens is moved to the focus servo control possible position, for example, the focus position (focus search control), and then, the focus servo control is carried out according to a focus error signal.
Since the level (peak value) of the focus error signal FE varies depending upon variations of discs and the temperature characteristic of an optical pickup, the focus servo control device does not fix the focus servo gain. It does detect the focus servo signal level, determines the attenuator value based on the detected level, and automatically obtains an optimum focus servo gain. This determination of the focus servo gain is performed simultaneously with the focus search control.
In other words, a focus search signal FS, which is a triangular wave signal of (1/2)Hz, is generated in a focus search as shown in FIG. 5, and the object lens is vertically moved in response to the focus search signal so as to come close to and then move away from the disc surface. In this vertical movement, the peak level L.sub.P of a focus error signal FE when the object lens first passes near the focus position (t.sub.1) is detected, the attenuator value is set based on the detected peak level, and the focus serve gain is adjusted. Furthermore, after the object lens reaches the focus position for a second time (t.sub.2), the focus servo control is performed according to the focus error signal FE instead of the focus search signal FS.
However, such a conventional method has the disadvantage in that it takes a period corresponding to one cycle of a focus search signal (2 seconds) to prepare for focus servo control since it is necessary to determine the focus servo gain, and thus sound is not output immediately. Although it is possible to shorten the cycle of the focus search signal in order to eliminate the above disadvantage, in this case, the moving velocity of the object lens in transition from the focus search to the focus servo control is made higher. In other words, the signal width of the focus error signal is made smaller and it is not possible to reliably prepare for the focus servo control.